Using slang is not itself a sign of being semiliterate, but knowing only the slang definitions of many words may be.
Consistently using the wrong spelling of similar-sounding words such as "their", "there", and "they're" is another common sign of poor literacy skills.
Perhaps the most frustrating sign of semiliteracy is the use of 'netspeak' or 'txt-speak', wherein the writer abbreviates short and simple words for no reason beyond laziness.
Semiliterate slang: This test is crucial because it's too hard!"
English: "They're going over there to get their equipment."
Semiliterate misuse: "Their going over their to get their equipment."
English: "Why are you writing like that?"
Semiliterate netspeak: "y r u riten lyke dat??"